Percussion drilling apparatus has been previously devised for downhole drilling operations and, for example, reference is made to U.S. Pat. No. 4,883,133 for Combustion Operated Drilling Apparatus granted to the applicants of this invention. According to our prior patent, concentric rows of drill rods having impact teeth at the lower ends of the rods are driven sequentially into the earth by firing a series of concentric combustion chambers above the drill rods and which requires closely controlled sequential firing of each combustion chamber to drive an individual piston or impact member downwardly into engagement with the drill rods.
In accordance with the present invention, it is proposed to employ a single combustion chamber with associated drive member or piston to sequentially drive the drill rods into the earth through a novel and improved form of impact block assembly which is interposed between the drive member and drill rods. This approach obviates close timing and control over a series of combustion chambers and shifts the sequential control over advancement of the drill rods to a separate impact block assembly.